Voltage controlled oscillators (VCOs) are generally used in high-frequency communication or sensor systems, for example, as an oscillator for a frequency synthesizer or a modulator within a wireless communication apparatus. As illustrated in prior art FIG. 1, a voltage controlled oscillator 10 receives an input or tuning voltage 12 and outputs a signal 14 having a frequency that is a function of the input voltage 12 to a load 16. Consequently, by altering the voltage at the input 12, one may tune the oscillation frequency at the output 14.
In many instances, the performance of a telecommunications device is limited by the phase noise of the oscillator circuit, wherein phase noise may be described generally as the random variation in the phase or frequency of the output signal. Consequently, the phase noise may place a limit on the precision of the oscillator output frequency. On the other hand, an oscillator with a reduced phase noise indicates that the oscillator produces a lower amount of spurious energy outside the desired frequency range of operation.
One type of voltage controlled oscillator is the inductive/capacitive (LC) oscillator. One example of an LC oscillator is illustrated in prior art FIG. 2, wherein a schematic diagram of a high frequency differential output LC type voltage controlled oscillator 20 is provided. The LC oscillator 20 is sometimes referred to as a negative-resistance oscillator because an input impedance between a base node of the transistors T and a virtual or AC ground has a negative real component and a capacitive reactance. The input impedance is influenced by a variable capacitance or varactor CVAR that facilitates a tuning of the oscillation frequency at the differential outputs Q and Q(bar). The inductance LB substantially completes the resonant circuit. The inductance LC matches the external load RL to the output of the transistor T to maximize the signal amplitude.
Circuit 20 of prior art FIG. 2 provides for some improvements over traditional millimeter-wave LC oscillators due to the inclusion of inductances LE1 and LE2. LE2 facilitates the feeding of the bias current from the transistor T into a virtual ground or AC ground node, and consequently an output capacitance of a transistor current source 22 is not shunted to CVAR and thus prevents a reduction in the tuning range of the oscillator. In addition, the contribution of the current source 22 to the circuit's phase noise is reduced dramatically. LE2 contributes to this advantage, together with the output capacitance of the current source 22, forms a low-pass LC filter that decouples high frequency noise generated by the current source 22 from the oscillator signal.
The addition of LE1 increases the loaded quality factor QL of the oscillator circuit 20 and thus reduces the phase noise thereof compared to a circuit having only a varactor CVAR. In addition, the inductances LE1, LE2 and the capacitance CVAR are designed so that the admittance YE of the total LC network that loads the emitter node of the transistor T is substantially capacitive within the entire oscillator frequency range of interest. The varactor CVAR in one example may comprise a base-collector type P-N junction that may be modified by an additional implant to provide a wised capacitance range. In addition, the inductances may comprise micro-strip lines formed in the metallization layers of an integrated circuit. The lengths of the micro-strip lines may be adjusted by clipping or cutting shorting bars associated therewith, thus altering the magnitude of the inductance.
The oscillator circuit 20 has an output matching network 24 that serves to optimize the load impedance at the collector nodes of the transistors T with respect to maximum output power and optimal loaded quality factor QL. The matching network 24 includes LC, a bond pad capacitance CP and, if the circuit is mounted, a bond inductance LQ. The matching network 24 couples to the external load RL.
In some applications such an oscillator is employed as part of a monolithically integrated circuit or in a multi-chip module where the oscillator output Q and Q(bar) are directly bonded to a loading chip. In such instances, an output buffer is not required. However, if the oscillator output is loaded by a transmission line and such line is (as usual) not perfectly terminated, the performance of the oscillator circuit 20 is degraded. For example, such performance degradation may be evidenced by discontinuities in a plot of oscillation frequency versus varactor bias voltage (not shown) within the desired frequency operating range.
One solution to decouple the oscillator circuit 20 from the load is illustrated in prior art FIG. 3, wherein a base-grounded output buffer 30 is coupled to the collector terminals of the transistors T (forming a cascode stage). The output buffer 30 of prior art FIG. 3 provides some measure of decoupling of the oscillator circuit 20 from the load, however, the buffer 30 suffers from several disadvantages: there is no substantial signal amplification, the decoupling capability is often not sufficient at very high frequencies, it reduces the oscillation amplitude and the loaded quality factor of the oscillator core, thus increasing the phase noise, and a higher supply voltage is required.
Therefore there is a need in the art for improved voltage controlled oscillator systems and output buffers that overcome the deficiencies of the prior art.